The present invention relates to data cables employing twisted pairs of insulated conductors as the transmission medium, and to cable splines for use in the data cables.
High performance twisted pair cables have become popular for a variety of reasons. Such cables are comparatively easy to handle, install, terminate and use. They also are capable of meeting high performance standards.
Commonly, multiple twisted pairs are used in these types of cables. In each pair, the wires are twisted together in a helical fashion forming a balanced transmission line. When twisted pairs are placed in close proximity, such as in a cable, electrical energy may be transferred from one pair of the cable to another. Such energy transfer between pairs is undesirable and is referred to as crosstalk. Crosstalk causes interference to the information being transmitted through the twisted pair and can reduce the data transmission rate and can cause an increase in the bit error rate. The Telecommunications Industry Association (TIA) and Electronics Industry Association (EIA) have defined standards for crosstalk in a data communications cable such as the Category 6 cable standard ANSI/TIA/EIA-568-B.2-1, published Jun. 20, 2002 by TIA. The International Electrotechnical Commission (IEC) has also defined standards for data communications cable crosstalk, such as ISO/IEC 11801, which includes the international equivalent to ANSI/TIA/EIA-568-B.2-1.
One popular cable type meeting the above specifications is foil shielded twisted pair (FTP) cable. FTP cable is popular for local area network (LAN) applications because it has good noise immunity and a low level of radiated emissions.
Another popular cable type meeting the above specifications is unshielded twisted pair (UTP) cable. Because it does not include shield conductors, UTP cable is preferred by installers and plant managers as it is easily installed and terminated. The requirements for modern state of the art transmission systems require both FTP and UTP cables to meet very stringent requirements. Thus, FTP and UTP cables produced today have a very high degree of balance and impedance regularity. In order to achieve this balance and regularity, the manufacturing process of FTP and UTP cables may include twisters that apply a back torsion to each wire prior to the twisting operation. Therefore, FTP and UTP cables have very high impedance regularities due to the randomization of eventual eccentricities in a twisted wire pair during manufacturing.
Crosstalk is primarily capacitively coupled or inductively coupled energy passing between adjacent twisted pairs within a cable. Among the factors that determine the amount of energy coupled between the wires in adjacent twisted pairs, the center-to-center distance between the wires in the adjacent twisted pairs is very important. The center-to-center distance is defined herein to be the distance between the center of one twisted pair to the center of an adjacent twisted pair. The center of a twisted pair may be taken as the point equidistant from and on the line passing through the center of each of the individual wires in the pair. The magnitude of both capacitively coupled and inductively coupled crosstalk varies inversely with the center-to-center distance between wires, approximately following an inverse square law. Increasing the distance between twisted pairs will thus reduce the level of crosstalk interference. Another factor affecting the strength of the coupling between two twisted pairs is the medium through which the wires couple and the electromagnetic properties of that medium. Examples of these properties include conductivity, permittivity, permeability, and loss tangent. Yet another important factor relating to the level of crosstalk is the distance over which the wires run parallel to each other. Twisted pairs that have longer parallel runs will have higher levels of crosstalk occurring between them.
In twisted pairs, the twist lay length is the longitudinal distance between twists of the wire. The direction of the twist is known as the twist direction. If adjacent twisted pairs have the same twist lay length, then the coupling is longitudinally additive. In other words, the crosstalk tends to be higher between pairs having substantially the same twist lay length. In addition, cables with the same twist lay length tend to interlink. Interlinking occurs when two adjacent twisted pairs are pressed together filling any interstitial spaces between the wires comprising the twisted pairs. Interlinking will cause a decrease in the center-to-center distance between the wires in adjacent twisted pairs and can cause a periodic coupling of two or more twisted pairs. This can lead to an increase in crosstalk among the wires in adjacent twisted pairs within the cable.
Therefore, adjacent twisted pairs within a cable are given unique twist lay lengths and the same twist directions. The use of unique twist lay lengths serves to decrease the level of crosstalk between adjacent twisted pairs. However, it causes the coupling strength between each possible pair of twisted-pairs in a cable to be different.
Additionally, if each adjacent twisted pairs in cable has a unique twist lay length and/or twist direction, other problems may occur. In particular, during use mechanical stress may interlink adjacent twisted pairs.
In order to obtain yet better crosstalk performance in FTP and UTP cables, for example, to meet performance standards such as the Category 6 standard, some have introduced an interior support or spline for the data cable, such as disclosed by Gaeris et al. in U.S. Pat. No. 5,789,711, issued Aug. 4, 1998, and by Gareis in U.S. Pat. No. 6,297,454, issued Oct. 2, 2001. Additional examples of such interior support for data cables are given by Prudhon in U.S. Pat. No. 5,952,615, issued Sep. 14, 1999, and also by Blouin et al. in U.S. Pat. No. 6,365,836, issued Apr. 2, 2002. Such splines serve to separate adjacent twisted pair cables and prevent interlinking of twisted pairs.
Conventional splines have the basic cross form, such as shown in FIG. 1. These shapes have a number of disadvantages, discussed below.
The conventional cable configuration of FIG. 1 includes a cable spline 101, a plurality of twisted pairs 102 of insulated conductors 103. Cable spline 101 has walls 104 with straight, parallel sides. The entire assembly is surrounded by a jacket (not shown) and possibly by a shield (optional, not shown).
During the stranding operation, the walls 104 of cable spline 101 may be stressed and thinned, allowing the twisted pairs 102 to move tangentially to the circumference of the cable in addition to radially, away from the center of the cable. This movement is undesirable, as it causes crosstalk and attenuation variation. Due to the latter, impedance also varies, exhibiting some roughness. Variation in crosstalk over time and distance is influenced by variations in center to center distance caused by tangential displacements of the twisted pairs over time and distance. The tangential displacement varies the spacing between pairs. Radial displacement predominantly affects attenuation. Variation in radial displacement cause attenuation variation, also called attenuation roughness, as the distance from the center of each twisted pair to the jacket varies. Both of these variations also incidentally have an impact upon impedance roughness.
In conventional cables, the loss factor or loss tangent of the jacketing material also has a substantial impact upon the attenuation figure of data grade cables. Attenuation increases with proximity of the transmission media to the jacket. For this reason, data cables not having an interior support such as disclosed by Gaeris et al. generally have loose fitting jackets. The looseness of the jacket reduces the attenuation figure of the cable, but introduces other disadvantages. For example, the loose fitting jacket permits the geometric relationship between the individual twisted pairs as well as the center-to-center distance to vary, thus varying impedance and crosstalk performance.
In FTP cable, the effect of the loss tangent of the jacketing material is substantially mitigated by the shield. The shielding characteristics of the foil surrounding the twisted pairs determine the effect upon different frequencies. This shielding characteristic is best described by the transfer impedance. However, measurement of the transfer impedance is difficult, especially at higher frequencies.
The performance of shielded cable can be substantially improved by individually shielding the twisted pairs. However, such cables commonly designated as STP (Individually Shielded Twisted Pairs) wires are impractical, as they require a substantial amount of time and specialized equipment or tools for termination. Additionally, the cables themselves are relatively large in diameter due to the added bulk of the shield. Bulkier cables exhibit poor flammability performance, and also occupy more space in ducts and on cross connects than less bulky cables.
The cable spline structures disclosed by Blouin et al. in U.S. Pat. No. 6,365,836, issued Apr. 2, 2002, solves the problem of attenuation due to loss tangent by increasing the distance between the twisted pairs and the cable jacket. The cable splines disclosed by Blouin, cross sections of which are shown in FIGS. 2 and 3, feature flanged walls 201 and 301 which extend sufficiently far around the twisted pairs 202 and 302 to retain them in a stable position, but also leave a groove for the insertion of the twisted pairs during manufacture. The voids formed in the splines for holding the twisted pairs may have a variety of cross-sectional shapes, as demonstrated in FIGS. 2 and 3.
While the structure described in Blouin solves the problems associated with loss tangent and controlling attenuation variation, it is still desirable to further reduce the losses due to crosstalk between twisted pairs. One method of reducing the crosstalk between twisted pairs is described by Gareis in U.S. Pat. No. 6,297,454. FIG. 4 shows an example of the spline disclosed by Gareis. Gareis makes use of a cable separator spline 401 having four walls 402–405 of the same shape and thickness, in which two 402 and 403 walls form a pair which are separated from the remaining two walls 404 and 405 by a fifth wall or bridge 406, causing the cable to have a minor axis 407 and a major axis 408. In this way two voids are formed which are separated by a distance which is greater than the distance separating the remaining two voids. Gareis teaches that the two voids separated by the greater distance have a radius which is less than the remianing two voids. By placing the two twisted pairs with the highest crosstalk in the voids which are separated by the greatest distance, better performance can be achieved.
In addition to suffering from the previously described problems of loss tangent, the cable spline disclosed by Gareis also introduces problems due to its shape. The elliptical shape of the cable introduces difficulties in spooling the cable, and also during installation. For example, it is desirable to spool cables as tightly as possible; to spool cables tightly, it is necessary to control their position during the spooling process. This process is made difficult when the cable does not have a circular cross-section, and may require additional time or equipment. In addition, non-circular cables may require special treatment during installation or greater pull strength due to having a preferential bend axis.
Additionally, it is desirable to further improve the cross-talk properties over the cables and cable splines previously discussed.